Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance of unrecognized tax benefits	$ 112	$ 107	$ 145
Additions related to acquisitions	0	22	7
Additions based on tax positions taken in current year	10	9	9
Additions based on tax positions taken in prior years	9	0	34
Reductions based on tax positions taken in prior years	0	(1)	(51)
Reductions resulting from statute of limitation lapses	(8)	(3)	(12)
Reductions related to settlements with taxing authorities	(8)	(22)	(25)
Ending balance of unrecognized tax benefits	$ 115	$ 112	$ 107